Oil field operators drill boreholes into subsurface reservoirs to recover oil and other hydrocarbons. After the primary production phase, at least some percentage of oil typically remains in the reservoir. In order to recover the residual oil in the reservoir, improved oil recovery techniques are applied. One particular improved oil recovery technique consists of injection of water, steam or chemical into the reservoir via secondary (“injection”) wells to encourage the oil to move to the primary (“production”) wells and thence to the surface. This injection could be monitored to minimize contamination of the oil in the production well. In conjunction with the permanent monitoring of production and injection wells, some decisions can be made to maximize the oil production and minimize the contamination. These decisions could include modifying injection parameters, such as reducing or stopping the injection, or modifying production parameters, such as flow rate at certain depth intervals, based on the monitoring measurements.
Furthermore, information about the geologic formations and about reservoir characteristics promotes efficient development and management of hydrocarbon resources. Reservoir characteristics include, among others, resistivity of the geologic formation containing hydrocarbons. The resistivity of geologic formations is generally related to porosity, permeability, and fluid content of the reservoir. Because hydrocarbons are generally electrically insulating and most formation water is electrically conductive, formation resistivity (or conductivity) measurements are a valuable tool in determining the hydrocarbon content of reservoirs. Moreover, formation resistivity measurements may be used to monitor changes in reservoir hydrocarbon content during production of hydrocarbons.
Generally, formation resistivity is sensitive to fluid saturation. The ability to remotely determine and monitor formation resistivity is of direct relevance to reservoir monitoring, particularly for enhanced recovery with water-flooding and/or carbon dioxide injection. Accordingly, there is continued interest in the development of permanent sensing systems.